MaterialDesignTheme.ToolBarTray.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Style x:Key="MaterialDesignToolBarTray" TargetType="{x:Type ToolBarTray}">
        <Setter Property="Background" Value="{DynamicResource MaterialDesignToolBarBackground}"/>
    </Style>
</ResourceDictionary>